A battery separator and its preparation method

Abstract

This invention discloses a battery separator and its preparation method, belonging to the field of battery separator preparation technology. The preparation method includes: providing a homogeneous melt containing a polyolefin and a mixed plasticizer; extruding the homogeneous melt into a cast sheet that is easy to stretch; and stretching the cast sheet to obtain a porous polyolefin separator, wherein the mixed plasticizer is removed by evaporation at atmospheric pressure during the stretching process; at least a portion of the components in the mixed plasticizer have boiling points higher than the extrusion temperature, and the mixed plasticizer contains a first component, and at least one of a second component and a third component, wherein the first component is n-dodecane, the second component is at least one of C8, C9, C10, and C11 straight-chain alkanes, and the third component is a C13 and / or C14 straight-chain alkanes. This invention uses a specific plasticizer, which can be directly recovered by atmospheric pressure evaporation during the stretching process, thereby not only reducing the environmental impact of using extractants, but also effectively improving the production efficiency and pore quality of the separator.

Description

Technical Field

[0001] This invention belongs to the field of battery separator preparation technology, and more specifically, relates to a battery separator and its preparation method. Background Technology

[0002] Lithium-ion batteries primarily function by the shuttle of lithium ions between the positive and negative electrodes. During charging and discharging, lithium ions exchange by inserting and de-intercalating between the electrodes. The separator, one of the four crucial elements of a lithium-ion battery, exists between the positive and negative electrodes, separating the materials and preventing short circuits. The separator's performance determines the battery's interface properties, directly affecting its capacity and safety. The quality of the separator significantly impacts the overall battery performance. Currently, the main battery separators used are PE, PP, and PP / PE / PP composite separators produced by Celgard (USA) and UBE (Japan) using a dry uniaxial stretching process.

[0003] Current wet-process manufacturing steps for lithium-ion battery separators include melt blending, cooling casting, stretching into a film, extraction pore formation, and heat setting. The extraction pore formation step utilizes a large amount of dichloromethane (LC) as an extractant to displace the oil phase in the oil film. Dichloromethane is a volatile organic solvent, and its recovery during separator manufacturing is difficult and costly. Furthermore, the loss of dichloromethane and its environmental impact throughout the process are significant. Therefore, it is crucial to develop high-performance microporous membranes while avoiding the use of dichloromethane as an extractant.

[0004] The applicant has been committed to improving the performance and manufacturing process of membranes, and has achieved certain research results in avoiding the use of dichloromethane extractant. For example, the applicant's application No. 2023105347633, filed in 2013, discloses a membrane and its preparation method. This application replaces the extraction step in the wet process by performing vacuum evaporation on a polyolefin membrane containing a pore-forming agent, thereby reducing the amount of organic solvent used, reducing the impact on the environment and human health, and increasing the membrane production rate. However, this application requires vacuum evaporation after membrane stretching, making the process operation relatively complex. Summary of the Invention

[0005] 1. Technical problems to be solved The purpose of this invention is to provide a battery separator and its preparation method, thereby solving the problems of existing wet separator preparation processes, which typically require extraction of pore-forming agents, resulting in relatively complex processes, reduced production speed, and environmental impact. This invention uses a specific plasticizer and directly evaporates and recovers the plasticizer during the stretching process, replacing the solvent extraction step in existing wet processes. This not only improves separator production efficiency and reduces environmental impact but also effectively ensures the pore quality of the separator.

[0006] 2. Technical Solution To achieve the above objectives, the technical solution provided by the present invention is as follows: The first aspect of this invention provides a method for preparing a battery separator, comprising: Provides homogeneous melts containing polyolefins and mixed plasticizers; Homogeneous melts are extruded and shaped into easily stretchable cast sheets; and The cast sheet is stretched to obtain a porous polyolefin membrane, and the plasticizer mixed in during the stretching process is removed by evaporation; All of the above operations are carried out under normal pressure, and at least some components of the mixed plasticizer have boiling points higher than the extrusion processing temperature. It includes a first component and at least one of a second component and a third component. The first component is n-dodecane, the second component is at least one of C8, C9, C10, and C11 straight-chain alkanes (i.e., octane, nonane, decane, and n-undecane), and the third component is at least one of C13 and C14 straight-chain alkanes (i.e., n-tetane and n-tetradecane).

[0007] To address the problems of existing wet-process membrane manufacturing, which requires extraction of pore-forming agents, resulting in relatively complex production processes, low production efficiency, and environmental impact, application number 2023105347633 proposes replacing the extraction process with vacuum evaporation of the stretched membrane, thus effectively solving the aforementioned technical problems. However, on the one hand, this process is performed after stretching, limiting its effect on improving membrane production efficiency; on the other hand, this application requires strict evaporation conditions for the pore-forming agent, necessitating vacuum evaporation. Based on the above, the applicant of this application continued to conduct research on the basis of the aforementioned patent. By adding a plasticizer with a specific composition, the above-mentioned processes can be carried out directly under normal pressure. In particular, the plasticizer can be directly evaporated and removed during the stretching process, without the need for a special extraction process. This greatly improves the production efficiency of the diaphragm and has lower requirements for the removal conditions of the plasticizer. It can be directly evaporated under normal pressure without changing the original stretching process conditions and the structure of the stretching process equipment.

[0008] It should also be noted that the selection of the type of mixed plasticizer in this application is crucial. Besides considering the compatibility between the plasticizer and the polyolefin and meeting the evaporation temperature requirements of the plasticizer, the volatilization rate of the plasticizer and the pore quality of the diaphragm also need to be considered. Specifically, at least some components of the mixed plasticizer preferably have boiling points higher than the extrusion processing temperature and are volatile within the stretching process temperature range, to ensure that the mixed plasticizer is mainly evaporated and removed during the stretching process. Meanwhile, the boiling point of plasticizers also affects their evaporation rate and the pore-forming quality of the membrane. When the boiling point of plasticizers is too low, their evaporation rate increases. During the stretching and pore-forming stage, the plasticizers evaporate too quickly. Although this is beneficial for improving the removal effect of plasticizers and reducing the amount of plasticizer residue, it will lead to a decrease in the pore-forming quality of the membrane, excessive porosity, and a decrease in the strength of the membrane. On the other hand, when the boiling point of plasticizers is too high, although it is beneficial for improving the pore-forming quality of the membrane and reducing the porosity of the membrane, the plasticizers evaporate more slowly, resulting in excessive plasticizer residue in the membrane, which in turn affects the performance of the membrane.

[0009] Therefore, based on the above considerations, this application employs a mixed plasticizer, using n-dodecane as the base component, and adding a second component (at least one of C8, C9, C10, and C11 straight-chain alkanes) and / or a third component (at least one of C13 and C14 straight-chain alkanes) as regulating components. This allows for the direct evaporation and removal of most of the plasticizer during the stretching process, while effectively ensuring both the removal efficiency of the plasticizer and the pore quality of the membrane. Specifically, the inventors of this application discovered during their research that n-dodecane not only has good compatibility with polyolefins, but also, although its boiling point is above 200 degrees Celsius (215~217℃), it can volatilize at around 110~120℃ without depressurization. Therefore, this application uses it as the main plasticizer component. Adding a second component to n-dodecane not only improves the volatilization rate of the plasticizer during the stretching process, ensuring effective plasticizer removal and reducing plasticizer residue in the diaphragm, but also better controls the relationship between the plasticizer and the air and temperature fields during stretching, resulting in products that meet technical requirements; it can also reduce production costs to some extent. Furthermore, adding a third component to n-dodecane can better adjust the pore morphology and permeability of the diaphragm product, thus further improving its performance.

[0010] In one embodiment, the mixed plasticizer comprises one of the second and third components and the first component, and the mass of the second and third components does not exceed 35% of the total amount of the mixed plasticizer. That is, the mixed plasticizer of this application can be based on the first component with only the second or third component added, and other types of plasticizers can also be added as needed. However, the amount of the second and third components added needs to be strictly controlled to prevent affecting the pore quality of the diaphragm or the residual amount of plasticizer in the diaphragm.

[0011] In another embodiment, the mixed plasticizer comprises a first component, a second component, and a third component. The combination of these three components ensures both effective plasticizer removal and good pore formation in the membrane. More preferably, the mixed plasticizer consists only of the first, second, and third components.

[0012] Furthermore, when the mixed plasticizer simultaneously contains the first component, the second component, and the third component, the sum of the masses of the second component and the third component shall not exceed 50% of the total amount of the mixed plasticizer. When the amount added is too much, it will affect the compatibility between the plasticizer and the polyolefin, and will also have an adverse effect on the removal effect of the plasticizer or the pore formation quality of the membrane.

[0013] According to any of the technical solutions described in the first aspect of the present invention, the mass percentage of the mixed plasticizer in the homogeneous melt, that is, the percentage of the sum of the mass of the mixed plasticizer and the polyolefin, is 40% to 90%, for example, a numerical range of 40% to 85%, 50% to 80%, 80% to 85%, etc., or specific values ​​such as 40%, 60%, 75%, 80%, 85%, etc. can be used.

[0014] According to any of the technical solutions described in the first aspect of the present invention, the crystallinity of the polyolefin obtained by extrusion molding is determined by DSC to be 26%~30%. Compared with traditional white oil pore-forming agents, the plasticizer of this application has better compatibility with polyolefin, that is, stronger interaction with polyolefin, and higher crystallinity of polyolefin in the plasticizer system, which is conducive to the formation of more crystals. According to any of the technical solutions described in the first aspect of the present invention, the volatilization rate of the mixed plasticizer during the stretching process is 69-75%. Specifically, the volatilization rate of the mixed plasticizer during the stretching process can be adjusted to a certain extent by controlling the component ratio in the mixed plasticizer according to actual needs. Therefore, most of the mixed plasticizer in the present invention can be directly evaporated and removed at atmospheric pressure during the stretching process, and the residual plasticizer can be basically volatilized during the subsequent heat setting process, thereby ensuring the removal effect of the plasticizer.

[0015] According to any of the technical solutions described in the first aspect of the present invention, the polyolefin is selected from homopolymers or copolymers of polyethylene and / or homopolymers or copolymers of polypropylene, specifically, homopolymers or copolymers of polyethylene, homopolymers or copolymers of polypropylene, or a mixture of both may be used.

[0016] More preferably, the molecular weight of the polyethylene homopolymer or copolymer is 700,000 to 2,000,000, and / or the solid content is 10% to 40%. The molecular weight and solid content of the polyolefin both affect the thickness and structural strength of the resulting diaphragm. Under the same process conditions, a higher molecular weight and higher solid content of the polyolefin result in higher structural strength of the diaphragm, but also an increased diaphragm thickness. Furthermore, process parameters such as the stretch ratio can be further optimized as needed to balance the relationship between diaphragm thickness and strength.

[0017] According to any of the technical solutions described in the first aspect of the present invention, the process of extruding the homogeneous melt into a casting that is easy to stretch is wherein the extrusion temperature is 170°C to 210°C, preferably 180°C to 200°C, and more preferably 190°C to 200°C.

[0018] According to any of the technical solutions described in the first aspect of the present invention, the stretching of the casting sheet specifically involves using a bi-stretching device to stretch the casting sheet sequentially, with a stretching temperature of 105 ℃ to 125 ℃.

[0019] According to any of the technical solutions described in the first aspect of the present invention, the method further includes: cooling the extruded casting and then performing a stretching process, wherein preferably a chilling roller is used for cooling, and the cooling temperature is approximately 10~20°C. According to any of the technical solutions described in the first aspect of the present invention, the method further includes: heat-setting the porous polyolefin separator obtained after stretching, wherein the plasticizer content in the finished porous polyolefin separator obtained after heat-setting is less than 1%.

[0020] A second aspect of the present invention provides a battery separator, which is prepared by any of the methods described in the first aspect of the present invention.

[0021] According to any of the technical solutions described in the second aspect of the present invention, the plasticizer content in the battery separator is less than 1%.

[0022] According to any of the technical solutions described in the second aspect of the present invention, the diaphragm has a thickness of 7-16 μm, an air permeability of 110-210 s / 100 mL, a porosity of 35-55%, a needle penetration strength of 250 gf-450 gf, and a longitudinal tensile strength ≥1400 kgf / cm². 2 Transverse tensile strength ≥1400 kgf / cm 2The longitudinal thermal shrinkage rate at 105 ℃×1 h is ≤ 5%, and the transverse thermal shrinkage rate is ≤ 3%.

[0023] A third aspect of the present invention also provides a battery separator, wherein the plasticizer content in the battery separator is less than 1%, the air permeability of the separator is 110-210 s / 100 mL, the porosity is 35-55%, the needle penetration strength is 250 gf~450 gf, and the longitudinal tensile strength is ≥1400 kgf / cm. 2 Transverse tensile strength ≥1400 kgf / cm 2 The longitudinal thermal shrinkage rate is ≤5% and the transverse thermal shrinkage rate is ≤3% at 105 ℃×1 h.

[0024] 3. Beneficial effects Compared with the prior art, the technical solution provided by this invention can achieve the following beneficial effects: (1) The present invention selects plasticizers that have good compatibility with polyolefins and are volatile, and stretches them together after extrusion molding. During the stretching process, most of the plasticizers can be evaporated and removed under normal pressure. Compared with the traditional wet membrane preparation process, the present application eliminates the extraction process, which not only improves the production efficiency and consistency of the membrane, but also facilitates the recycling of plasticizers and can effectively ensure the recycling effect of plasticizers, thus achieving high production efficiency and high output.

[0025] (2) By optimizing the design of the components of the mixed plasticizer and strictly controlling the amount of each component added, this invention can not only ensure the compatibility between the plasticizer and the polyolefin and meet the evaporation temperature requirements of the plasticizer, but also effectively coordinate the volatilization rate of the plasticizer and the pore quality of the membrane.

[0026] (3) The residual amount of plasticizer in the diaphragm prepared by the present invention is less than 1%, the residual amount of plasticizer is low, and the mechanical strength, air permeability, pore quality and heat shrinkage performance of the diaphragm are all excellent. Attached Figure Description

[0027] Figure 1 This is a flowchart of the membrane preparation process of the present invention; Figure 2 DSC curves of PE with molecular weights of 70 w and 130 w versus plasticizer in white oil and dodecane systems; Figure 3 TG curves of the dodecane system at different stretching temperatures; Figure 4 DSC curves for the white oil and dodecane system (plasticizer): (a) First heating curve; (b) Cooling crystallization curve; (c) Second heating curve; Figure 5DSC curves of white oil and dodecane system (plasticizer) after plasticizer removal: (a) First heating curve; (b) Cooling crystallization curve; (c) Second heating curve; Figure 6 DSC curves of crystallization of white oil and dodecane system (plasticizer) at different cooling rates: (a) white oil system; (b) dodecane system; Figure 7 The phase diagrams for the white oil and dodecane system (plasticizer) at different cooling rates are shown. Detailed Implementation

[0028] This application describes a process involving mixing, melt extrusion, cooling, and stretching of polyolefins with a mixed plasticizer to obtain a porous polyolefin membrane. The mixed plasticizer comprises a first component and at least one of a second and a third component. The first component is n-dodecane, the second component is at least one of C8, C9, C10, and C11 straight-chain alkanes, and the third component is at least one of C13 and C14 straight-chain alkanes. At least some components of the mixed plasticizer have boiling points (see Table 1 below for specific boiling points of each plasticizer) higher than the extrusion processing temperature, exhibiting good compatibility with polyolefins. By using this mixed plasticizer, most of the plasticizer can be removed directly by atmospheric pressure evaporation during the stretching process without subsequent extraction. This avoids the environmental and human health hazards associated with using large amounts of organic solvents, increases production line speed, reduces costs, and does not require changes to the original stretching process conditions. It also reduces potential scratches on the membrane caused by rollers in the original wet extraction section, improving membrane quality.

[0029] Table 1 Boiling Points of Each Plasticizer

[0030] To fully demonstrate the good compatibility between the plasticizer system of this application and polyolefins, n-dodecane is used as an example to conduct a comparative study with the traditional white oil system.

[0031] like Figure 2 The figure shows the DSC curves of PE with molecular weights of 70 w and 130 w mixed with plasticizer in the white oil system and the dodecane system. The white oil system uses white oil as the plasticizer mixed with PE, while the dodecane system uses C12 plasticizer mixed with PE. As can be seen from the figure, when the molecular weight of PE is 70 w, its melting point is similar in both the white oil system and the dodecane system (C12 system). However, the melting point of PE with a molecular weight of 130 w in the C12 system is about 3.8 °C lower than that in the white oil system, indicating that the C12 system plasticizer has better compatibility with PE.

[0032] The melting and crystallization process of C12 and ultra-high molecular weight PE (UHMWPE) mixed castings was characterized using DSC. Meanwhile, a mixed casting of white oil and UHMWPE was used as a control sample to compare and analyze the compatibility behavior between the C12 system and existing white oil plasticizers with PE. The results are as follows: Figure 4 As shown, where Figure 4 (a) shows the DSC curve of the melt during the first heating process of the extruded casting. Figure 4 (b) is the cooling crystallization curve. Figure 4 (c) shows the melting curve after the second heating following cooling and crystallization. Figure 4 As shown in Table 2 below, in the C12 system, the melting temperature of PE in the cast sheet is about 5 °C lower than that in the white oil system; during the cooling crystallization process, the crystallization temperature of PE in the C12 system is about 10 °C lower than that in the white oil system. This indicates that the C12 system interacts more strongly with PE, making PE less prone to crystallization and requiring crystallization under lower temperatures and greater supercooling conditions. Figure 4 As can be seen from the second heating DSC curve in (c), the melting temperature of PE in the C12 system is also lower than that of the white oil system. However, in terms of crystallinity, although the melting temperature of PE in the C12 system is lower than that of the white oil system, the crystallinity of PE in the C12 system is higher than that of the white oil system, meaning that the C12 system is more conducive to the formation of more crystals.

[0033] Table 2 Melting point and crystallinity of white oil system and C12 system

[0034] To further verify the effect of plasticizer on PE, C12 and white oil were removed from the casting. DSC was used to test the melting and crystallization of PE after plasticizer removal. The results are as follows: Figure 5 As shown. Combined with Figure 5 It can be seen that after removing the plasticizer, the melting temperature of PE in the original white oil system is close to that in the original C12 system. This indicates that the presence of the C12 system is more conducive to the melting of PE crystals, meaning that C12 and PE have stronger compatibility. During the cooling crystallization process, the crystallization temperature of PE in the original white oil system after removing the white oil is no significantly different from that in the original C12 system, with a difference of <1℃. This suggests that the lower crystallization temperature of PE in the C12 system is due to the interaction between C12 and PE, rather than factors inherent to PE itself.

[0035] To reveal the phase separation characteristics, DSC was used to characterize the differences in crystallization behavior of the C12 / PE mixture and the white oil / PE mixture under different cooling rates. The results are as follows: Figure 6 As shown, where Figure 6 (a) shows the cooling crystallization curves of white oil and PE mixed at different cooling rates of 2, 5, 10, and 50 ℃ / min. Figure 6(b) shows the cooling crystallization curves of C12 and PE mixture at different cooling rates. The initial crystallization temperature was statistically analyzed, and the crystallization phase diagrams at different cooling rates were plotted. Figure 7 As shown in the figure, under the same cooling rate, the initial crystallization temperature of PE in the C12 system is about 10 °C lower than that in the white oil system. At a slower cooling rate of 2 °C / min, the initial crystallization temperature of PE in the white oil system is around 110 °C, while that in the C12 system is around 100 °C. At an even faster cooling rate of 50 °C / min, the initial crystallization temperature of PE in the white oil system drops to 100 °C, while that in the C12 plasticizer system drops to 88 °C.

[0036] To further understand the content of this invention, it will now be described in detail with reference to specific embodiments.

[0037] Example 1 like Figure 1 As shown, 25 g of material with an average molecular weight of 7 × 10⁻⁶ is used. 5 Ultra-high molecular weight polyethylene (UHMWPE) was used as the polyolefin, and a mixture of 100 g of n-dodecane and n-tetradecane (mass ratio 2:1) was added to the feeding system as a plasticizer. The solid content of the UHMWPE and plasticizer mixture was 20%. The mixture was stirred at 60 r / min to obtain a homogeneous mixture. The homogeneous mixture was then added to an extruder, and the extrusion processing temperature was set to 190 ℃, the speed to 200 r / min, and continuous extrusion was performed to obtain uniform cast sheets.

[0038] The extruded sheet was cooled by a chiller roller to approximately 11°C. A bi-stretching device was then used to stretch the cooled sheet sequentially at 115°C, with a stretching ratio of 7 times. During stretching, pores were created to obtain a porous membrane. Because the temperature during stretching met the boiling point of the plasticizer, the plasticizer could effectively volatilize under normal pressure. The membranes before and after stretching were weighed, and the weight of the membranes during stretching was recorded to determine the efficiency of plasticizer volatilization.

[0039] Table 3 below shows the solvent evaporation rate data of the diaphragm after bistretching at different stretching temperatures in Example 1. At a stretching temperature of 110°C, the solvent evaporation rate of the diaphragm is 71.51%; at a stretching temperature of 120°C, the solvent evaporation rate of the diaphragm is 71.35%; and at a stretching temperature of 130°C, the solvent evaporation rate of the diaphragm is 70.96%. The solvent evaporation rates of the three types of diaphragms are not significantly different, indicating that the plasticizer inside the sample has basically evaporated completely after stretching.

[0040] Table 3. Volatilization efficiency of plasticizer in Example 1 at different stretching temperatures

[0041] like Figure 3 The figure shows the TG curves at different stretching temperatures in Example 1. As can be seen from the figure, the diaphragm obtained in Example 1, after stretching and heat setting, has a mass reduction of about 0.5% at 400°C, indicating that the residual amount of plasticizer in the diaphragm after stretching and heat setting is less than 1%.

[0042] Example 2 25 g of material with an average molecular weight of 7×10 5 Ultra-high molecular weight polyethylene (UHMWPE) was used as the polyolefin, and a mixture of 100 g of n-dodecane and n-octane (mass ratio 2:1) was added to the feeding system as a plasticizer. The solid content of the UHMWPE and plasticizer mixture was 20%. The mixture was stirred at 60 r / min to obtain a homogeneous mixture. The homogeneous mixture was then added to an extruder, with the extruder temperature set at 190 ℃ and the speed at 200 r / min, and continuously extruded to form uniform cast sheets.

[0043] The extruded sheet is cooled by a chiller roller to a temperature of approximately 11 °C. A bi-stretching device is then used to stretch the cooled sheet sequentially at a temperature of 125 °C, with a stretching ratio of 7 times. During stretching, pores are created to obtain a porous membrane. Because the temperature during stretching meets the boiling point of the plasticizer, the plasticizer effectively volatilizes under normal pressure.

[0044] Example 3 25 g of material with an average molecular weight of 7×10 5 Ultra-high molecular weight polyethylene (UHMWPE) was used as the polyolefin, and a mixture of 100 g of n-dodecane, n-tetradecane, and n-octane (mass ratio 4:1:1) was added to the feeding system as a plasticizer. The solid content of the UHMWPE and plasticizer mixture was 20%. The mixture was stirred at 60 r / min to obtain a homogeneous mixture. The homogeneous mixture was then added to an extruder, with the extruder temperature set to 180 ℃ and the speed set to 200 r / min, and continuously extruded to form uniform cast sheets.

[0045] The extruded sheet is cooled by a chiller roller to a temperature of approximately 11 °C. A bi-stretching device is then used to stretch the cooled sheet sequentially at a temperature of 120 °C, with a stretching ratio of 8 times. During stretching, pores are created to obtain a porous membrane. Because the temperature during stretching meets the boiling point of the plasticizer, the plasticizer effectively volatilizes under normal pressure.

[0046] Example 4 25 g of material with an average molecular weight of 13×10 5Ultra-high molecular weight polyethylene (UHMWPE) was used as the polyolefin, and a mixture of 142 g of n-dodecane and n-tetradecane (mass ratio 2:1) was added to the feeding system as a plasticizer. The solid content of the UHMWPE and plasticizer mixture was 15%. The mixture was stirred at 60 r / min to obtain a homogeneous mixture. The homogeneous mixture was then added to an extruder, with the extruder temperature set at 190 ℃ and the speed at 200 r / min, and continuously extruded to form uniform cast sheets.

[0047] The extruded sheet is cooled by a chiller roller to approximately 11°C. A bi-stretching device is then used to sequentially stretch the cooled sheet at 115°C with a stretching ratio of 7. During stretching, pores are created to form a porous membrane. Because the temperature during stretching meets the boiling point of the plasticizer, the plasticizer effectively volatilizes.

[0048] Example 5 25 g of average molecular weight 9×10 5 Ultra-high molecular weight polyethylene (UHMWPE) was used as the polyolefin, and a mixture of 37.5 g of n-dodecane and n-tridecane (mass ratio 3:1) was added to the feeding system as a plasticizer. The solid content of the UHMWPE and plasticizer mixture was 40%. The mixture was stirred at 80 r / min to obtain a homogeneous mixture. The homogeneous mixture was then added to an extruder, with the extruder temperature set at 190 ℃ and the speed at 200 r / min, and continuously extruded to form uniform cast sheets.

[0049] The extruded sheet is cooled by a chiller roller to approximately 14°C. A bi-stretching device is then used to sequentially stretch the cooled sheet at 125°C with a stretching ratio of 6. During stretching, pores are created to form a porous membrane. Because the temperature during stretching meets the boiling point of the plasticizer, the plasticizer effectively volatilizes.

[0050] Example 6 25 g of material with an average molecular weight of 1.5 × 10⁻⁶ 6 Ultra-high molecular weight polyethylene (UHMWPE) was used as the polyolefin, and a mixture of 142g of n-dodecane, decane, and n-undecane (mass ratio 10:1:1) was added to the feeding system as a plasticizer. The solid content of the UHMWPE and plasticizer mixture was 15%. The mixture was stirred at 60 r / min to obtain a homogeneous mixture. The homogeneous mixture was then added to an extruder, with the extruder temperature set at 195 ℃ and the speed at 210 r / min, and continuously extruded to form uniform cast sheets.

[0051] The extruded sheet was cooled by a chiller roller to approximately 10 °C. A bi-stretching device was then used to stretch the cooled sheet sequentially at 105 °C, with a stretching ratio of 7 times. During stretching, pores were created to obtain a porous diaphragm. Because the temperature during stretching met the boiling point of the plasticizer, the plasticizer effectively volatilized. The diaphragms before and after stretching were weighed, and the weight of the diaphragms during stretching was recorded to determine the efficiency of plasticizer volatilization.

[0052] Example 7 25 g of average molecular weight 8×10 5 Ultra-high molecular weight polyethylene (UHMWPE) was used as the polyolefin, and a mixture of 100 g of n-dodecane, nonane, and n-tridecane (mass ratio 4:1:1) was added to the feeding system as a plasticizer. The solid content of the UHMWPE and plasticizer mixture was 20%. The mixture was stirred at 60 r / min to obtain a homogeneous mixture. The homogeneous mixture was then added to an extruder, with the extruder temperature set at 175 ℃ and the speed at 200 r / min, and continuously extruded to form uniform cast sheets.

[0053] The extruded sheet is cooled by a chiller roller to approximately 10°C. A bi-stretcher is then used to stretch the cooled sheet sequentially at 122°C with a stretch ratio of 7. During stretching, pores are created to form a porous membrane. Because the temperature during stretching meets the boiling point of the plasticizer, it effectively volatilizes. The membranes before and after stretching are weighed, and the weight of the membranes during stretching is recorded to determine the efficiency of plasticizer volatilization.

[0054] Comparative Example 1 The main difference between this comparative example and Example 1 is that only n-dodecane is added as a plasticizer.

[0055] Comparative Example 2 The main difference between this comparative example and Example 1 is that only n-tetradecane is added as a plasticizer.

[0056] Comparative Example 3 The main difference between this comparative example and Example 1 is that only n-octane is added as a plasticizer.

[0057] Comparative Example 4 The main difference between this comparative example and Example 1 is that the mass ratio of the mixed plasticizer is different from that in Example 1. In this comparative example, the mass ratio of n-dodecane to n-tetradecane is 1:1.

[0058] Comparative Example 5 The main difference between this comparative example and Example 2 is that the mass ratio of the mixed plasticizer is different from that in Example 2. In this comparative example, the mass ratio of n-dodecane to n-octane is 1:1.

[0059] Comparative Example 6 The main difference between this comparative example and Example 3 is that the mass ratio of the mixed plasticizer is different from that in Example 3. In this comparative example, the mass ratio of n-dodecane, n-tetradecane, and n-octane is 1:1:1.

[0060] Diaphragm performance testing The test items and methods are as follows: 1. Thickness Testing equipment: Millimar C1216 thickness gauge (Germany) Test method: The electrical signal fed back by the sensor probe is converted into a numerical output display to measure the actual thickness of the diaphragm under test.

[0061] 2. Porosity Testing equipment: Mercury porosimeter Test method: Mercury is used as the test solution. Under pressure, the mercury is squeezed into the pores of the membrane to be tested. The pressure corresponding to the mercury squeezed into different pore sizes follows the Washburn equation. By controlling different pressures, the volume of mercury squeezed into the pores can be measured, thereby obtaining the cumulative distribution curve or calculus curve corresponding to different pressures and pore sizes.

[0062] Washburn equation: r = 2σ cosθ / p In the formula: r is the capillary pore diameter (m); σ is the surface tension of mercury (mN / m); θ is the contact angle between mercury and the capillary surface; p is the applied external pressure (mN / m). 2 ).

[0063] 3. Breathability Testing equipment: Digital Wangyan-type air permeability meter Test method: The diaphragm to be tested is placed on the test platform, with one side sealed by the probe end face, and the other side is sealed by the probe end face. During the test, the air pressure changes continuously due to the characteristics of the test material. The instrument uses an internal permeability sensor to calculate the time required for gas to pass through 100 cc of unit area.

[0064] 4. Needle puncture intensity Testing equipment: KES-G5 pressure tester Test method: The test is conducted using a Hall force sensor. The internal amplifier outputs a voltage value, which is converted into a numerical signal by an integrator. The terminal displays the test value of the needle penetration intensity of the diaphragm under test.

[0065] 5. Tensile strength Testing equipment: Intelligent electronic tensile testing machine, model AI-3000-S, High-speed rail testing instruments (Dongguan) Co., Ltd.

[0066] Test method: The intelligent electronic tensile testing machine uses a screw clamping belt and gears to lift or lower the sample. The sample undergoes tensile testing, and the data is displayed and analyzed by a microcomputer to obtain data from the force sensor.

[0067] 6. Heat shrinkage Test equipment: Electric heating drying oven, model DHG-9053A, Shanghai Yiheng Scientific Instruments Co., Ltd.; Test method: The diaphragm to be tested is reacted under a set temperature condition for a period of time. The difference in length between the transverse and longitudinal directions of the diaphragm is measured with a ruler, and the thermal shrinkage ratio is calculated.

[0068] Table 4. Performance test results of the diaphragm in each embodiment and comparative example.

[0069] Table 4 shows some compositional parameters and performance test results of the membranes obtained in Examples 1-7 and Comparative Examples 1-6. The comparative results of Comparative Examples 1-3 indicate that when only a single plasticizer (such as C12, C14, or C8) is used, it is impossible to simultaneously and effectively match the plasticizer's volatility, the pore quality of the resulting membrane, and the mechanical properties of the membrane (needle penetration strength, tensile strength, etc.). C12 exhibits good compatibility with polyolefins. Failure to add C12, or insufficient addition of C12, leads to poor compatibility between the plasticizer and polyolefins, affecting the mechanical properties of the resulting membrane.

[0070] Based on C12, by adding a second component (such as C8) and strictly controlling the amount of the second component, the volatilization rate of the plasticizer in the stretching process can be effectively adjusted without significantly affecting the compatibility between the plasticizer and the polyolefin material, as well as the mechanical properties of the resulting diaphragm and other performance characteristics. However, excessive addition of the second component can lead to excessively large pores and a loose structure. At the same time, by replacing part of C12 with C8, the amount of C12 used can be effectively reduced, which is beneficial to saving production costs.

[0071] By adding a third component (such as C14) to C12, the pore quality of the membrane can be controlled, which helps to optimize the pore morphology. Compared with compounding with the second component alone, compounding C12 with the third component can further improve the pore quality and mechanical properties (needle penetration strength, tensile strength, etc.) of the resulting membrane. However, the amount of the third component added should not be too high, otherwise it will inhibit the volatilization of plasticizer and affect the pore quality and mechanical properties of the membrane. By compounding C12 with the second and third components, not only can the removal effect of plasticizer in the stretching process be guaranteed, but the pore quality and mechanical properties of the resulting membrane can also be improved, thus comprehensively coordinating the relationship between the three components.

Claims

1. A method for preparing a battery separator, characterized in that, include: Provides homogeneous melts containing polyolefins and mixed plasticizers; Homogeneous melts are extruded and shaped into cast sheets that are easy to stretch; as well as The cast sheet is stretched, and during the stretching process, the plasticizer is mixed in and removed by evaporation, thereby obtaining a porous polyolefin membrane; All of the above operations are carried out under normal pressure, and at least some components of the mixed plasticizer have boiling points higher than the extrusion processing temperature. It includes a first component and at least one of a second component and a third component. The first component is n-dodecane, the second component is at least one of C8, C9, C10, and C11 straight-chain alkanes, and the third component is at least one of C13 and C14 straight-chain alkanes.

2. The method for preparing the battery separator according to claim 1, characterized in that, The mixed plasticizer comprises one of the second component and the third component, as well as the first component, and the mass of the second component and the third component does not exceed 35% of the total mass of the mixed plasticizer.

3. The method for preparing the battery separator according to claim 1, characterized in that, The mixed plasticizer comprises a first component, a second component, and a third component.

4. The method for preparing the battery separator according to claim 3, characterized in that, The combined mass of the second and third components shall not exceed 50% of the total mass of the mixed plasticizer.

5. The method for preparing the battery separator according to any one of claims 1-4, characterized in that, The mass percentage of the mixed plasticizer in the homogeneous melt is 40% to 90%.

6. The method for preparing the battery separator according to any one of claims 1-4, characterized in that, The volatilization rate of the plasticizer during the stretching process is 69-75%.

7. The method for preparing the battery separator according to any one of claims 1-4, characterized in that, The polyolefin is selected from homopolymers or copolymers of polyethylene and / or homopolymers or copolymers of polypropylene.

8. The method for preparing the battery separator according to claim 7, characterized in that, The homopolymer or copolymer of the polyethylene has a molecular weight of 700,000 to 2,000,000 and / or a solid content of 10% to 40%.

9. The method for preparing the battery separator according to any one of claims 1-4, characterized in that, The process of extruding a homogeneous melt into a cast sheet that is easy to stretch is described, wherein the extrusion temperature is 170 ℃~200 ℃; and / or The stretching of the casting sheet specifically involves using a bi-stretching device to stretch the casting sheet sequentially, with a stretching temperature of 105 ℃~125 ℃.

10. The method for preparing the battery separator according to any one of claims 1-4, characterized in that, Also includes: The extruded castings are cooled and then stretched. and / or The porous polyolefin separator obtained after stretching is heat-set, and the plasticizer content in the finished porous polyolefin separator after heat-setting is less than 1%.

11. A battery separator, characterized in that, The diaphragm is prepared by the method described in any one of claims 1-10.

12. The battery separator according to claim 11, characterized in that, The plasticizer content in the battery separator is less than 1%; and / or The diaphragm has a thickness of 7-16 μm, an air permeability of 110-210 s / 100mL, a porosity of 35-55%, a needle penetration strength of 250 gf-450 gf, and a longitudinal tensile strength ≥1400 kgf / cm². 2 Transverse tensile strength ≥1400 kgf / cm 2 The longitudinal thermal shrinkage rate at 105 ℃×1 h is ≤ 5%, and the transverse thermal shrinkage rate is ≤ 3%.

13. A battery separator, characterized in that, The battery separator contains less than 1% plasticizer, has a thickness of 7-16 μm, an air permeability of 110-210 s / 100mL, a porosity of 35-55%, a needle penetration strength of 250 gf-450 gf, and a longitudinal tensile strength ≥1400 kgf / cm². 2 Transverse tensile strength ≥1400 kgf / cm 2 The longitudinal thermal shrinkage rate at 105 ℃×1 h is ≤5%, and the transverse thermal shrinkage rate is ≤3%.

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